1. Field of the Invention
The present invention relates to a printer preferably used in, for example, a facsimile.
2. Description of the Prior Art
Generally, a line-type thermal head is provided in a facsimile, in which a document image transmitted from the other facsimile is thermally recorded on a thermal sensitive paper by the line-type thermal head. Such line-type thermal head includes a plurality of exothermic elements aligned on a line, respective exothermic elements being selectively heated in response to print data.
In a conventional facsimile, the exothermic elements in one line are divided into several groups, for example, into eight groups, and the exothermic elements in respective groups are heated and controlled successively. Strobe signals S1 to S8 and the print data DA for heating the exothermic elements of respective groups in such facsimile are shown in FIG. 1.
The strobe signals S1 to S8 correspond separately to a plurality of groups divided into eight. Each exothermic element in respective groups is heated in response through the data signal DA when the corresponding strobe signal Si (i=1.about.1) is in a low level.
That is, as shown in FIG. 1 (9), the data signal DA includes, for example, data DA1 for one line in a period Wa, and as shown in FIG. 1 (10), the data DA1 is a serial signal representing in logic "1" and "0", whether or not each exothermic element of respective groups is heated in the period Wa1 to Wa8. The data DA for one line is stored, for example, in a shift register or the like. Each cell of the shift register corresponds separately to each exothermic element.
At start of a period Wc, data stored in the shift register are latched respectively in a latch circuit having cells corresponding separately to respective cells of the shift register. In the period Wc, the strobe signals S1 through S8 are pulse signals which become low level in sequence. While the strobe signals Si corresponding to respective groups are low level, the exothermic elements to be heated in the groups are heated in response to the latch circuit data.
At the start of a period Wd following the period Wc, the data DA1 of the shift register is latched in the latch circuit. In the period Wd, the strobe signals S1 through S8 become low level successively the same as in the period Wc previously described, and respective exothermic elements are heated in response to the data DA1 to thereby effect printing. In a period Wb within the period Wd, print data DA2 of the next line are led out and stored in the shift register. By repeating such operations while the thermal sensitive paper is transferred, image information transmitted from the other facsimile are recorded on the thermal sensitive paper.
Meanwhile, there is a fervent desire to speed up the communication speed of image information of the facsimile, thus manufactures of the facsimile, besides a communication system standardized by CCITT (a Consulting Committee of International Telegraph and Telephone), decide their own communication system to manufacture the facsimile capable of communicating the image information more speedily. As the communication speed of the image information is increased as such, it is indispensable to improve the printing speed in the facsimile.
In the printing system described above, in order to improve the printing speed without deteriorating printing quality, there is a method of reducing the heating time of respective exothermic elements by employing a low resistance value of each exothermic element and applying a large current to each exothermic element. In this method, such a problem is encountered that an expensive exothermic element having a low resistance value is required. Though a power circuit supplying the current for heating the exothermic element is constructed to supply the current sufficient to heat all of the exothermic elements in one group, when improving the printing speed in this method, the power circuit must be designed to supply the larger current.
In another method, printing is executed by dividing the exothermic elements constituting the line-type thermal head into small groups, for example, into four groups. In this case, in order to maintain the printing quality, since the total number of exothermic elements can not be reduced and a number of exothermic elements are included in each group, the number of exothermic elements being heated simultaneously increases. Accordingly, the power circuit must still be designed to supply the large current.
As such, in the two methods aforementioned, a powerful power circuit is required to supply a predetermined current to the exothermic elements, resulting in an expensive and large-sized apparatus.